Crosslinked hybrid gate dielectric materials and electronic devices incorporating same

ABSTRACT

Disclosed are thin film transistor devices incorporating a crosslinked inorganic-organic hybrid blend material as the gate dielectric. The blend material, obtained by thermally curing a mixture of an inorganic oxide precursor sol and an organosilane crosslinker at relatively low temperatures, can afford a high gate capacitance, a low leakage current density, and a smooth surface, and can be used to enable satisfactory transistor device performance at low operating voltages.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to and the benefit of U.S. ProvisionalPatent Application Ser. No. 61/346,245, filed on May 19, 2010, thedisclosure of which is incorporated by reference herein in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Grant NumberN00014-05-1-0541 awarded by the Office of Naval Research, Grant NumberDMR-0520513 awarded by the National Science Foundation, and Grant NumberFA9550-08-1-0331 awarded by the Air Force Office of Scientific Research.The government has certain rights in the invention.

BACKGROUND

Organic thin-film transistors (OTFTs)-based electronics performingsimple operations offer unique attractions compared to conventionalinorganic technology, including low-cost large-area coverage, and lowprocessing temperatures suitable for flexible substrates. A typical OTFTincludes a number of layers and can be configured in various ways. Forexample, a bottom-gate top-contact OTFT includes a substrate with a gateelectrode thereon, a dielectric layer deposited over the gate electrode,a semiconductor layer in contact with the dielectric layer, and sourceand drain electrodes separated from each other and deposited on thesemiconductor layer. In this device architecture, the current betweenthe source electrode and the drain electrode is modulated by both thesource-drain voltage (V_(SD)) and the source-gate voltage (V_(G)). Whenthe device is in the off-state (V_(G)=0V), the channel current is verylow, whereas in the on-state of the source-gate voltage (V_(G)≠0V),large current increase is observed. The saturation current in organicthin-film transistors is generally described by Equation 1

$\begin{matrix}{{I_{DS} = {\frac{W}{2L}\mu\;{C_{i}\left( {V_{G} - V_{T}} \right)}^{2}}},} & (1)\end{matrix}$where, μ is the field-effect charge carrier mobility, C_(i) is thecapacitance per unit area of the dielectric, V_(T) is the thresholdvoltage, and W and L are the OTFT channel width and length,respectively. Despite recent impressive progress of new organicsemiconductors, large OTFT operating voltages, reflecting theintrinsically low mobilities of organic semiconductors compared toconventional inorganic semiconductors, remain one of the majorchallenges to overcome. For low power applications such as RFID,displays, and portable electronics, it is mandatory to achieve high TFTdrain currents (I_(SD)) at acceptably low operating voltages. Withoutchanging device geometry (W and L) and semiconductor material (μ),equivalent OTFT I_(SD) can be achieved at lower operating voltages byincreasing the gate dielectric capacitance C_(i), given by Equation 2

$\begin{matrix}{{C_{i} = {ɛ_{0}\frac{k}{d}}},} & (2)\end{matrix}$where ∈₀ is the vacuum permittivity, k is the dielectric constant, and dis the thickness of dielectric layer. From Equation 2, it can be seenthat operating bias reduction can be achieved by increasing thedielectric constant (k) or decreasing the thickness (d) of the gatedielectric. An increase of the k/d ratio is also essential for efficientdevice scalability, a prerequisite to improving low-power TFT operation.

Gate electric materials that can be processed by solution and at lowtemperatures are important to enable compatibility with flexible plasticsubstrates. Crosslinked polymer films, inorganic metal oxides,polymer/high-k nanoparticle composites and hybrid organic/inorganicdielectrics have been investigated as candidates for low-voltage TFTs.However, many of these dielectrics have limitations in achievingpractical applications for flexible low-voltage TFTs. For example,crosslinked polymer materials have relatively low-k values and thus TFTdrain currents (I_(SD)) at low operating voltages often are notsufficiently high. An alternative approach is to employ high-k materialssuch as metal oxide (MO) films. However, high-quality MO dielectricfilms often require high growth/annealing temperatures (>400° C.) orvacuum conditions (atomic layer or chemical/physical vapor deposition)to ensure low leakage currents. Furthermore, most high-k metal oxidematerials, particularly crystalline metal oxide materials, are often toobrittle for flexible applications. Another method to increase the k/dratio and mechanical flexibility is to use polymer-high k inorganicnanoparticle composites. However, because the dielectric constant ofthese composite materials is dominated by the relatively low-k polymercomponent, a large nanoparticle load is necessary to increase the kvalue of the composite, resulting in increased surface roughness.Lastly, although hybrid gate dielectrics composed of self-assembledmonolayers or multilayers on ultrathin inorganic oxides show promise forlow-voltage OTFTs, their integration into large-volume coating processescan be difficult.

Therefore, there is a need in the art for dielectric materials that canbe prepared at low temperatures via solution-phase processes and thatcan enable low-voltage TFTs.

SUMMARY

In light of the foregoing, the present teachings relate to a crosslinkedinorganic-organic hybrid blend material which can have good film anddielectric properties, and can be used as a gate dielectric in bothorganic and inorganic thin film transistors to enable satisfactorydevice performance at low operating voltages (e.g., <±10 V). Thecrosslinked hybrid blend (CHB) dielectric material generally can beobtained by thermally curing a mixture of an inorganic oxide precursorsol and an organosilane crosslinker at relatively low temperatures asdetailed herein.

Accordingly, the present teachings also relate to thin film transistorsthat include a dielectric component comprising the crosslinkedinorganic-organic hybrid blend material described herein and methods offabricating such thin film transistors.

The foregoing as well as other features and advantages of the presentteachings will be more fully understood from the following figures,description, examples, and claims.

BRIEF DESCRIPTION OF DRAWINGS

It should be understood that the drawings described below are forillustration purposes only. The drawings are not necessarily to scale,with emphasis generally being placed upon illustrating the principles ofthe present teachings. The drawings are not intended to limit the scopeof the present teachings in any way.

FIG. 1 illustrates four different configurations of thin filmtransistors (TFTs): bottom-gate top-contact (A), bottom-gatebottom-contact (B), top-gate bottom-contact (C), and top-gatetop-contact (D) TFTs.

FIG. 2A shows X-ray reflectivity (XRR) plots for control ZrO₂ thin filmsfabricated on Si substrates and annealed at various temperatures.

FIG. 2B plots the XRR-derived film thickness of the control ZrO₂ thinfilms as a function of the annealing temperature.

FIGS. 3A and 3B show AFM images of crosslinked hybrid blend dielectricfilms (CBTH (A) and CBTO (B) films) obtained from a blend compositionhaving the indicated ZrCl₄:crosslinker molar ratio.

FIG. 4A shows leakage current density-electric field (J-E) plots for thecontrol ZrO₂ dielectric films annealed at 150° C., 300° C., and 400° C.

FIG. 4B shows representative transfer plots for pentacene OTFTs based onthe control ZrO₂ dielectrics of FIG. 4A.

FIGS. 5A and 5B plot capacitance as a function of voltage measured at 10kHz (left) and as a function of frequency measured at 3 V (right) forCBTH (A) and CBTO (B) films.

FIGS. 5C and 5D show leakage current density-electric field (J-E) plotsfor CBTH (C) and CBTO (D) films.

FIGS. 6A and 6B show transfer (left) and output (right) plots forpentacene OTFTs based on CHB dielectrics obtained with aZrCl₄:crosslinker molar ratio of 1:0.5 (A) and 1:0.2 (B), respectively.

FIGS. 7A and 7B show transfer (A) and output (B) plots for a pentaceneflexible OTFT based on a CBTH dielectric.

FIGS. 8A and 8B show X-ray diffraction patterns of pentacene filmsdeposited on CBTH (A) and CBTO (B) dielectrics with the indicatedZrCl₄:crosslinker molar ratios.

FIG. 9 shows AFM images of pentacene films on CBTH (A) and CBTO (B)dielectrics with the indicated ZrCl₄:crosslinker molar ratios. Imagesare 5 μm×5 μm in size.

FIG. 10 shows transfer plots for tin-doped indium oxide (ITO) TFTs basedon a CHB dielectric have a ZrCl₄:crosslinker molar ratio of 1:0.5.

DETAILED DESCRIPTION

Throughout the application, where compositions are described as having,including, or comprising specific components, or where processes aredescribed as having, including, or comprising specific process steps, itis contemplated that compositions of the present teachings also consistessentially of, or consist of, the recited components, and that theprocesses of the present teachings also consist essentially of, orconsist of, the recited process steps.

In the application, where an element or component is said to be includedin and/or selected from a list of recited elements or components, itshould be understood that the element or component can be any one of therecited elements or components, or can be selected from a groupconsisting of two or more of the recited elements or components.Further, it should be understood that elements and/or features of acomposition, an apparatus, or a method described herein can be combinedin a variety of ways without departing from the spirit and scope of thepresent teachings, whether explicit or implicit herein.

The use of the terms “include,” “includes”, “including,” “have,” “has,”or “having” should be generally understood as open-ended andnon-limiting unless specifically stated otherwise.

The use of the singular herein includes the plural (and vice versa)unless specifically stated otherwise. In addition, where the use of theterm “about” is before a quantitative value, the present teachings alsoinclude the specific quantitative value itself, unless specificallystated otherwise. As used herein, the term “about” refers to a ±10%variation from the nominal value unless otherwise indicated or inferred.

It should be understood that the order of steps or order for performingcertain actions is immaterial so long as the present teachings remainoperable. Moreover, two or more steps or actions may be conductedsimultaneously.

The present teachings, in part, provide a crosslinked inorganic-organichybrid blend material that can exhibit good electrically insulating ordielectric properties, which together with desirable mechanicalcharacteristics such as robustness and surface smoothness, render themuseful as dielectric components for various electronic devices, inparticular, low-voltage thin film transistors.

The present hybrid blend material can be prepared via solution-phasemethods with relatively low annealing temperatures. Specifically, theblend material can be obtained by thermally curing a precursor solution(or hereinafter “blend composition”) that includes an inorganic oxideprecursor sol and an organosilane crosslinker at a temperature less thanabout 300° C.

In various embodiments, the inorganic oxide precursor sol can beprepared by dissolving one or more metal oxide precursors such as metalchlorides, metal alkoxides, or combinations thereof, in a solvent orsolvent mixture. The metal oxide precursors can undergo various forms ofhydrolysis and polycondensation reactions upon annealing to form ametal-oxygen-metal lattice. In certain embodiments, the metal oxideprecursors can include a metal selected from Zr, Ti, Hf, and Ta. Inparticular embodiments, the precursor sol can be a zirconia sol whichincludes a zirconium compound such as ZrCl₄, ZrOCl₂, and Zr(OR)₄,wherein each R independently can be a C₁₋₆ alkyl group. To acceleratehydrolysis, the precursor sol can include a hydrolyzing catalyst such asan acid. The solvent or solvent mixture can include water and/or one ormore organic solvents, for example, various alcohols, aminoalcohols,carboxylic acids, glycols, hydroxyesters, aminoesters, and mixturesthereof. In certain embodiments, the solvent or solvent mixture can beselected from water, methanol, ethanol, propanol, butanol, pentanol,hexyl alcohol, heptyl alcohol, ethyleneglycol, methoxyethanol,ethoxyethanol, methoxypropanol, ethoxypropanol, methoxybutanol,dimethoxyglycol, N,N-dimethylformamide, and mixtures thereof.

The organosilane crosslinker typically is α,ω-functionalized with one ormore hydrolyzable groups. In various embodiments, the organosilanecrosslinker can be represented by the formula:(X)_(3-m)(Y)_(m)Si—Z—Si(Y)_(m)(X)_(3-m),wherein X, at each occurrence, independently can be selected from ahalide, an amino group, an alkoxy group, and a carboxylate group; Y, ateach occurrence, independently can be selected from H, an alkyl group,and a haloalkyl group; Z can be a divalent organic group comprising 1 to20 carbon atoms; and m, at each occurrence, independently can beselected from 0, 1, and 2. To illustrate, Z can be a divalent C₁₋₂₀alkyl group (linear or branched), a divalent C₁₋₂₀ haloalkyl group(where one and up to all of the hydrogen atoms in the alkyl group can bereplaced with a halide such as F or Cl), or a divalent C₁₋₂₀ alkyl groupwhere at least one of the CH₂ groups is replaced by O. In mostembodiments, Z can be an organic group that does not include easilyhydrolyzable bonds. In certain embodiments, X, at each occurrence,independently can be selected from F, Cl, —NR¹R², —OR³, and —OC(O)R³,where R¹ and R², at each occurrence, independently can be H or a C₁₋₆alkyl group, and R³, at each occurrence, independently can be selectedfrom H, a C₁₋₆ alkyl group, and a C₁₋₆ haloalkyl group. In particularembodiments, X, at each occurrence, independently can be selected fromCl, —OCH₃, —OCH₂CH₃, —OC(O)CH₃, and —OC(O)CH₂CH₃. In variousembodiments, m can be 0. In particular embodiments, the organosilanecrosslinker can be a bis(silyl)alkane comprising 4 to 10 carbon atoms,where each silyl group can be functionalized with at least one alkoxygroup. Specific examples of organosilane crosslinkers for practicing thepresent teachings include: [CH₃C—O]₃Si—(CH₂)₆—Si[O—CH₃]₃,[C₂H₅C—O]₃Si—(CH₂)₈—Si[O—C₂H₅]₃,[CH₃C(O)O]₃Si—(CH₂)₃—O—(CH₂)₂—O—(CH₂)₃—Si[CH₃C(O)O]₃,Cl₃Si—(CH₂)₃—O—(CH₂)₂—O—(CH₂)₃—SiCl₃,(CH₃O)₃Si—(CH₂)₂-phenyl-(CH₂)₂—Si(OCH₃)₃,Cl₃Si—(CH₂)₂—(CF₂)₂—(CH₂)₂—SiCl₃, and[CH₃C(O)O]₃Si—(CH₂)₂(CF₂)₂(CH₂)₂—Si[CH₃C(O)O]₃.

The organosilane crosslinker reacts with the inorganic oxide precursorsol as the inorganic oxide precursors undergo various hydrolysis andcondensation reactions to become an inorganic oxide gel, and uponannealing, the organosilane crosslinker and the inorganic componenttogether form a crosslinked insoluble network. Without wishing to bebound to any particular theory, it is believed thatalkoxy-functionalized organosilane crosslinker can lead to better filmproperties in some embodiments, due to the relatively slow condensationreaction between the alkoxy groups of the crosslinker and the hydroxylgroups of the inorganic oxide gel. For example, the resulting films canbe denser and more robust, hence providing improved leakage currentproperty. The distance between the two silyl groups in the organosilanecrosslinker also can contribute to slightly different film properties.For example, in embodiments where a bis(silyl)alkane is used as thecrosslinker, the alkane can have between about 5 to about 10 carbonatoms, preferably between about 6 to about 8 carbon atoms. The relativemolar ratio of the inorganic oxide precursor and the organosilanecrosslinker in the blend composition also can be modified to vary filmproperties. Generally, the metal oxide precursors are present in theblend composition at a molar concentration in excess of the organosilanecrosslinker. For example, the metal oxide precursors can be present inthe blend composition at a molar concentration between about 2 times andabout 10 times of the molar concentration of the organosilanecrosslinker.

The present crosslinked hybrid blend dielectric film can be prepared viavarious solution-phase deposition methods. Exemplary solution-phasedeposition methods include printing (e.g., inkjet printing and variouscontact printing techniques such as screen-printing, gravure printing,offset printing, pad printing, lithographic printing, flexographicprinting, and microcontact printing), spin-coating, drop-casting, zonecasting, dip coating, blade coating, spraying, rod coating, or stamping.In particular embodiments, the blend composition described herein can bespin-coated onto a substrate at a sufficient speed and for a sufficientperiod of time to achieve the desired thickness, followed by thermalcuring at relatively low temperatures. For example, the annealingtemperature can be less than about 400° C., less than about 300° C., orless than about 200° C. (e.g., about 150° C.), and can be carried out byvarious methods known in the art, for example, by using resistiveelements (e.g., ovens), IR radiation (e.g., IR lamps), microwaveradiation (e.g., microwave ovens), and/or magnetic heating. Inparticular embodiments, thermal curing of the blend composition can takeplace at a temperature between about 70° C. and about 150° C. in anatmosphere having a relative humidity ranging from about 5% to about95%. Instead of or in addition to thermal curing, the blend compositioncan be exposed to ultraviolet light to induce crosslinking. Typically,the thickness of the present crosslinked hybrid blend film can rangefrom about 10 nm to about 50 nm, although thicker films, if desired, canbe easily obtained with multiple spin-coating cycles.

The present crosslinked hybrid blend dielectrics can exhibit excellentinsulating properties including low leakage current densities (<about10⁻⁶ Acm⁻²), tunable capacitance (95˜385 nF cm⁻²), and a high dielectricconstant k (4.6˜8.7). The present crosslinked hybrid blend films alsoshow good surface morphologies and can form good interfaces with varioussemiconducting materials, including organic semiconducting compounds.

The desirable dielectric and interfacial properties of the presenthybrid blend dielectrics render them compatible with diverse groups ofsemiconductor materials (including both inorganic and organicsemiconductors) and suitable as dielectric materials in variouselectronic devices. Thin-film transistors fabricated with the presenthybrid blend materials as the dielectric material and various inorganicand organic thin film semiconductors can exhibit high mobilities andcurrent on/off ratios while enabling significantly reduced operatingvoltages (<±10 V) compared with conventional SiO₂ dielectrics (±100V).

Accordingly, in one aspect, the present teachings can relate to a methodof fabricating a thin film transistor. A thin film transistor can havedifferent configurations as shown in FIG. 1, including bottom-gatetop-contact structure (A), bottom-gate bottom-contact structure (B),top-gate bottom-contact structure (C), and top-gate top-contactstructure (D). A thin film transistor generally includes a substrate(12, 12′, 12″, and 12″), electrical conductors (source/drain conductors2, 2′, 2″, 2′″, 4, 4′, 4″, and 4′″, and gate conductors 10, 10′, 10″,and 10′″), a dielectric component 8, 8′, 8″, and 8′″ coupled to the gateconductor, and a semiconductor component 6, 6′, 6″, and 6′″ coupled tothe dielectric on one side and in contact with the source and drainconductors on the other side. As used herein, “coupled” can mean thesimple physical adherence of two materials without forming any chemicalbonds (e.g., by adsorption), as well as the formation of chemical bonds(e.g., ionic or covalent bonds) between two or more components and/orchemical moieties, atoms, or molecules thereof.

The present methods of fabricating a thin film transistor can includeforming a crosslinked hybrid dielectric film by thermally curing (e.g.,at a temperature less than about 300° C.) a blend composition depositedon a substrate, where the blend composition includes an inorganic oxideprecursor sol and an organosilane crosslinker; and forming a thin filmsemiconductor adjacent to the crosslinked hybrid blend dielectric film.The thin film semiconductor can be deposited by various methods known inthe art, including both vapor-phase methodologies (e.g., atomic layer orchemical/physical vapor deposition) and solution-phase methodologies(e.g., printing, spin-coating, drop-casting, zone casting, dip coating,blade coating, spraying, rod coating, or stamping).

In some embodiments, the thin film semiconductor can be a metal oxide.Exemplary semiconducting metal oxides include indium oxide (In₂O₃),tin-doped indium oxide (ITO), indium zincoxide (IZO), zinc tin oxide(ZTO), indium gallium oxide (IGO), indium-gallium-zinc oxide (IGZO), tinoxide (SnO₂), and zinc oxide (ZnO). In certain embodiments, the metaloxide thin film semiconductor can be deposited by a solution-phasemethod. In particular embodiments, the metal oxide thin filmsemiconductor can be formed by a sol-gel process.

In some embodiments, the thin film semiconductor can include one or moreorganic compounds, for example, one or more semiconducting moleculesand/or polymers. Exemplary semiconducting molecules and polymers includevarious fused heterocycles, aromatic hydrocarbons (e.g., pentacene),polythiophenes, fused (hetero)aromatics (e.g., perylene imide andnaphthalene imide small molecule or polymers), and other such organicsemiconductor compounds or materials, whether p-type or n-type,otherwise known or found useful in the art. In various embodiments, theorganic thin film semiconductor can be vapor-deposited, spin-coated, orprinted.

The method also can include forming source and drain electrodes incontact with the thin film semiconductor (for example, deposited on topof the thin film semiconductor for top-contact structures, or depositedon top of the hybrid multilayer dielectric layer for bottom-contactstructures). The gate electrode and the other electrical contacts(source and drain electrodes) independently can be composed of metals(e.g., Au, Ag, Al, Ni, Cu), transparent conducting oxides (e.g., ITO,IZO, ZITO, GZO, GIO, GITO), or conducting polymers (e.g.,poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS),polyaniline (PANI), or polypyrrole (PPy)). For embodiments where theelectrical contacts are composed of a metal, vacuum deposition can beused, typically through a shadow mask.

The substrate component for a thin film transistor can be selected fromdoped silicon, glass, aluminum or other metals alone or coated on apolymer or other substrate, as well as polyimide or other plastics. Incertain embodiments, the substrate can be a flexible plastic such as apolyimide film (e.g., KAPTON® from Dupont).

The following examples are provided to illustrate further and tofacilitate the understanding of the present teachings and are not in anyway intended to limit the invention.

EXAMPLE 1 Crosslinked Hybrid Blend (CHB) Dielectric Film Fabrication andCharacterization

Crosslinked hybrid blend (CHB) thin films were prepared from a blendsolution that includes an organosilane crosslinker and a zirconia sol.Specifically, zirconium (IV) chloride (99.5%, Aldrich) was dissolved inethanol (absolute >99.5%, Aldrich) to give a 0.1 M solution, to whichwas added a mixture of nitric acid and deionized water (molar ratio;ZrCl₄:HNO₃:H₂O=1:10:10). This zirconium precursor solution was thenheated to 50° C. for 3 h to accelerate hydrolysis to provide a zirconiasol. The organosilane crosslinker (bis(trimethoxysily)hexane, BTH (98%,Gelest), or bis(triethoxysily)octane, BTO (98%, Gelest)) was dissolvedin EtOH to provide a 0.1 M crosslinker solution, which was then mixedwith the zirconia sol in the following molar ratios: (a) 1:0.5, (b)1:0.2, and (c) 1:0.1 (ZrCl₄:crosslinker) to provide three differentblend solutions. Each of the blend solutions were filtered through a 0.2μm pore size PTFE membrane syringe filter before spin-coating. CHB thinfilms were deposited onto substrates by spin-coating at 5000 rpm for 30sec, followed by curing in a vacuum oven at 150° C. for 2 h.

Control zirconium oxide (ZrO_(x)) films were fabricated by spin-coatingthe zirconia sol (i.e., without the organosilane crosslinker) ontosubstrates at 5000 rpm for 30 sec, and then cured at 150° C., 300° C.,400° C., and 500° C. for 1 h.

The film thickness of the CHB dielectrics was analyzed with aprofilometer (Veeco Dektak 150 surface profiler). The film thickness ofthe control ZrO₂ dielectrics at different annealing temperatures wasanalyzed by X-ray reflectivity (XRR) using CuKα radiation (Rigaku ATX-GThin-film Diffraction Workstation). The morphologies of all thin filmswere evaluated by atomic force microscopy (AFM) using a JEOL-5200scanning probe microscope with silicon cantilevers in the tapping mode.

As derived from XRR data, the control ZrO₂ films obtained at anannealing temperature of 150° C., 300° C., 400° C., and 500° C.,respectively, have a film thickness of ˜17.0 nm, ˜12.1 nm, ˜11.0 nm, and˜10.9 nm (FIG. 2), indicating that higher annealing temperatures (>400°C.) lead to thinner and denser films. This is consistent with theunderstanding that for solution-processed metal oxide films, metalhydroxides are gradually converted into the oxides via thermally-drivencondensation processes (Zr—OH+OH—Zr→Zr—O—Zr+H₂O), and the degree ofoxide formation depends primarily on the annealing temperature.

Profilometry established that the CHB films have a film thickness in therange of about 20 nm to about 43 nm. Thicker films, if required, can beobtained by varying the processing conditions or by multiple spin-ondepositions (given that CHB films are insoluble in the mother solutionsat all stages of curing).

FIG. 3 shows AFM images of CBTH and CBTO films deposited from azirconium chloride/BTH blend solution and a zirconium chloride/BTO blendsolution, respectively. Independent of the ZrCl₄:crosslinker molarratio, BTH was found to afford very smooth crosslinked surfaces (RMSroughness, ρ˜0.2 nm). CHB films obtained with BTO have a higher RMSroughness (ρ=0.6-5 nm). Without wishing to be bound to any particulartheory, the longer hydrocarbon chain in BTO may lead to more foldingthan BTH, which may lead to relatively rougher surfaces. This may alsoexplain smoother films with a lower BTO molar ratio. These data suggestthat the surface smoothness of the CHB films can be optimized by varyingthe molar ratio of the organosilane crosslinker and/or the number ofcarbon atoms in the organosilane crosslinker.

The dielectric properties of both CHB films and the control ZrO₂ filmswere assessed by metal-insulator-semiconductor (MIS) leakage andcapacitance measurements. Metal-insulator-semiconductor (MIS) sandwichstructures were fabricated by directly depositing gold electrodes (200μm×200 μm) onto CHB or ZrO₂ dielectric-coated Si substrates through ashadow mask. MIS direct current measurements were carried out underambient conditions using a Signatone probestation using Keithley 6430Sub-Femtoamp Remote Source Meter and a Keithley 2400 source meter with alocal LabVIEW program. An impedance analyzer (HP 4192A) was used forcapacitance measurements.

FIG. 4A shows typical current density-electric field (J-E) plots for MISstructures fabricated with ZrO₂ dielectrics at different annealingtemperatures. It can be seen that the leakage current density of thesefilms increases as the annealing temperature decreases. Thus, highannealing temperatures (≧300° C.) are needed to obtain adequatedielectric strength (<10⁻⁶ A/cm² at 2 MV/cm) with sol-gel-derived ZrO₂thin films for typical OTFT operation (vide infra).

FIG. 5 shows the capacitance and leakage current density measurementsfor MIS structures fabricated with CBTH and CBTO dielectrics. It can beseen that the CBTH and CBTO devices exhibit leakage current densitiesthat are at least 10 times lower (<1×10⁻⁶ A/cm²) than the leakagecurrent densities exhibited by the ZrO₂ devices (˜1×10⁻⁵ A/cm²) at thesame bias window (2 MV/cm). Further, it was observed that the leakagecurrent density of the CHB devices decreases as the molar ratio of theorganosilane crosslinker increases. Without wishing to be bound to anyparticular theory, it is believed that this is the result of theimproved crosslinked network that affords denser films, as compared toinorganic-only films when annealed at low annealing temperature.However, CV measurements show that by increasing the molar ratio of theorganosilane crosslinker, the film capacitance decreases. Withoutwishing to be bound to any particular theory, it is believed that thisis the result of (1) the dilution of the inorganic component in theblend and (2) increased film thickness. It should be noted that comparedto the capacitance of a 300 nm thick SiO₂ dielectric (˜11 nF/cm²), thepresent CHB dielectrics can obtain much higher capacitance (95˜385nF/cm²). From the accumulation regime capacitances, k values of 4.6˜8.7are obtained for the two organosilane crosslinkers at the threedifferent molar ratios investigated. These values are quite comparablewith other inorganic materials, and are higher than conventional SiO₂dielectrics (3.9) as well as other dielectric materials based upon apolymer-inorganic nanoparticle nanocomposite (see e.g., Kim, P., Zhang,X.-H., Domercq, B., Jones, S. C., Hotchkiss, P. J., Marder, S. R.,Kippelen, B. and Perry, J. W. Appl. Phys. Lett., 93013302 (2008); Jung,C., Maliakal, A., Sidorenko, A. and Siegrist, T. Appl. Phys. Lett.,90062111 (2007); Maliakal, A., Katz, H., Cotts, P. M., Subramoney, S,and Mirau, P. J. Am. Chem. Soc., 127: 14655 (2005); and Schroeder, R.,Majewski, L. A. and Orrell, M. Adv. Mater., 17: 1535 (2005)). Asdemonstrated by the leakage current and capacitance measurements, theCHB films have good dielectric properties and can be used as dielectricmaterials, for example, in thin film transistor devices. Table 1 belowsummarizes various film and dielectric properties of the CHBdielectrics.

TABLE 1 Various film and dielectric properties for CHB and control ZrO₂dielectrics. Film Dielectric Cross- Zr:Cx T_(a) Thickness Film linker(Cx) ratio (° C.) (nm) J (A/cm²) ε ZrO₂ None 1:0 150 17.0 ~5 * 10⁻⁵ 9300 12.1 ~1 * 10⁻⁶ 10 400 11.0 ~3 * 10⁻⁸ 11 500 10.9 ~2 * 10⁻⁸ 11 CBTHBTH 1:0.5 150 35 ~4 * 10⁻⁷ 5.4 1:0.2 23 ~1 * 10⁻⁶ 7.1 1:0.1 19 ~4 * 10⁻⁶8.7 CBTO BTO 1:0.5 150 43 ~1 * 10⁻⁶ 4.6 1:0.2 27 ~3 * 10⁻⁶ 6.1 1:0.1 20~6 * 10⁻⁶ 8.3

EXAMPLE 2 Thin Film Transistor (TFT) Fabrication

Thin film transistors based upon different semiconductors and substratesand incorporating CHB films as the gate dielectric were fabricated.Control TFT devices were fabricated with ZrO₂ films as the gatedielectric. TFT measurements were carried out under ambient conditionsusing a Signatone probestation using Keithley 6430 Sub-Femtoamp RemoteSource Meter and a Keithley 2400 source meter with a local LabVIEWprogram. From the I-V data, the average field-effect mobility wascalculated in the saturation regime (V_(G)<V_(DS)=−4 V) by plotting thesquare root of the drain current versus gate voltage.

Using pentacene as a representative organic semiconductor, bottom-gate,top-contact OTFTs incorporating CHB films as the gate dielectric werefabricated on silicon substrates and plastic substrates, respectively,as follows. Control TFTs incorporating a ZrO₂ dielectric were fabricatedfollowing analogous procedures. In each case, the OTFTs have a channellength (L) of about 100 μm and a channel width (W) of about 2000 μm.

Heavily doped n⁺-type Si wafers (Montco Silicon Technologies, Inc.) werecleaned in EtOH (Aldrich, absolute, 200 proof) with sonication for 2 minand then dried with flowing nitrogen, followed by oxygen plasmatreatment for 5 min to remove organic contamination and to improve theirwetting ability. The CHB dielectric layer was deposited as described inExample 1. Pentacene was vacuum deposited onto the CHB-coated Sisubstrate (50 nm, 5×10⁻⁶ Torr 0.05 nm/s). Gold source/drain electrodeswere vacuum-deposited (50 nm, 0.02 nm/s) through a shadow mask.

Flexible OTFTs were fabricated by vacuum-depositing aluminum as the gateelectrode on a plastic substrate (KAPTON®), depositing the dielectriclayer as described in Example 1, vacuum-depositing pentacene (50 nm,5×10⁻⁶ Torr 0.05 nm/s) onto the dielectric-coated Al/KAPTON® substrate,and thermally evaporating gold (50 nm, 0.02 nm/s) as the source/drainelectrodes through a shadow mask.

Inorganic TFTs were fabricated with a solution-phase deposited indiumtin oxide (ITO) thin film semiconductor as follows. An indium tin oxideprecursor solution was first prepared by dissolving InCl₃ and SnCl₄ atvarious molar ratios [In³⁺:(In³⁺+Sn⁴⁺)=1 to 9, total metal concentration1 mM] in 1 mL of 2-methoxyethanol in a 2.5 mL capacity vial. To thissolution, ethanolamine (0.03 ml, 0.5 mmol) was added into each vial andthe clear solutions were then stirred for 30 min at room temperaturebefore spin-coating. CBTH-coated (ZrCl₄:crosslinker molar ratios=1:0.5)Si substrates were rinsed with absolute ethanol and dried with an N₂stream. The semiconductor was deposited by spin-coating the ITOprecursor solution onto these substrates at the speed of 3000 rpm for 30sec. Subsequently, the spin-coated films were annealed on a hotplate at250° C. for 1 hour. After cooling to room temperature, the spin-coatingprocess was repeated two times. Gold source/drain electrodes werevacuum-deposited (50 nm, 0.02 nm/s) through a shadow mask. The channellength (L) and width (W) are 100 μm and 2000 μm, respectively.

FIG. 4B shows the transfer plots of pentacene OTFTs based on ZrO₂control films which were annealed at 150° C.→400° C.). It can be seenthat the hole mobility improves from 0.1 cm²/V·s to 0.4 cm²/V·s as theannealing temperature increases from 150° C. to 400° C.

FIG. 6 shows representative transfer plots for OTFTs with the CHB gatedielectric annealed at 150° C. Table 2 summarizes the transistorcharacteristics of the CHB devices.

TABLE 2 Zirconium Chloride/Crosslinker Molar Ratio, Capacitance (C_(i),nF/cm²), Dielectric RMS Roughness (ρ, nm), OTFT Carrier Mobility(μ_(sat), cm²/Vs), and Current On/Off Ratio (I_(on):I_(off)) Data forOTFT/MIS Devices Fabricated Using P5 as the Organic Semiconductor andVarious ZrO₂ and CHB Dielectrics. Dielectric T_(a) P5 Film CrosslinkerRatio (° C.) C_(i) ρ μ_(sat) I_(on)/I_(off) V_(TH) ZrO₂ None 1:0 150 3900.2 0.1 10⁴ −0.7 300 567 0.2 0.3 10⁵ −1.0 400 700 0.2 0.4 10⁵ −1.1 500700 0.2 0.4 10⁵ −1.1 CBTH BTH 1:0.5 150 135 0.2 1.5 10⁵ −1.2 1:0.2 2750.2 1.0 10⁵ −1.1 1:0.1 385 0.2 0.7 10⁴ −1.0 CBTO BTO 1:0.5 150 95 0.60.2 10³ −1.8 1:0.2 200 1.3 0.3 10⁴ −1.5 1:0.1 365 5 0.6 10⁴ −1.1

Referring to the data in FIG. 6 and Table 2, it can be seen that OTFTsbased on CHB films require far lower annealing temperatures to operateproperly when compared to the ZrO₂ control devices. Specifically, theCHB devices all exhibit excellent linear and saturation regimecharacteristics and minimal hysteresis when operated at −4 V.Furthermore, because all CHB dielectric films are very smooth, each ofthe tested CHB devices exhibits satisfactory device performance. Inparticular, pentacene TFTs fabricated with the CBTH dielectric obtainedwith a ZrCl₄:BTH molar ratio of 1:0.5 show exceptional devicecharacteristics with a mobility of about 1.5 cm²/V·s, a current on/offratio of ˜10⁵, and a threshold voltage of about −1.2 V, respectively.TFTs based on CBTO also exhibit performance with μ approaching about 0.6cm²/V·s, I_(on/off)˜10⁴, and V_(TH)˜−1.1 V. These carrier mobilities arefar larger than those of control devices fabricated with a conventional300 nm thick SiO₂ gate dielectric (μ˜0.3 cm²/V·s). Further, theoperating voltage is only −4V versus −100V for SiO₂ as a result of thegreater capacitance of CHB films.

With continued reference to the data in Table 2, among the tested CHBdevices, it is suggested that optimum performance for CBTH devices wasobtained with a ZrCl₄:BTH molar ratio of 1:0.5, and for CBTO devicesZrCl₄:BTO=1:0.1. For CBTH devices, the gate capacitance was observed toincrease as the ZrCl₄:BTH molar ratio increases. However, the dielectricstrength was observed to decrease with increasing ZrCl₄:BTH molarratios, possibly due to the less dense films. In contrast, the CBTOdevices show improved performance with increasing ZrCl₄:BTO molar ratiosfrom 1:0.5 to 1:0.1, possibly as a result of the smoother filmmorphology.

FIG. 7 shows representative transfer and output plots for a flexiblepentacene TFTs based on CBTH (ZrCl₄:BTH molar ratio=1:0.5). These plotsdemonstrate reproducible I-V characteristics at low operating voltages(<−4 V) as well as excellent linear/saturation behavior. The holemobilities were measured to be ˜1.6±0.2 cm²/V·s and current on-offcurrent ratios ˜10⁵. These properties are quite comparable to the TFTsfabricated on silicon wafer substrates. FIGS. 8 and 9 show XRR patternsand AFM images of pentacene films deposited on the present CBTH and CBTOdielectrics, confirming that the CBTH and CBTO dielectrics have goodinterfacial properties.

FIG. 10 shows representative transfer plots of an inorganic TFT basedupon an ITO semiconductor and a CHB dielectric. The data confirm thatthese inorganic TFTs also operate well at low operating voltages (μ˜0.1cm₂/Vs and I_(on/off)>10²) and demonstrate the applicability of CHBdielectrics in hybrid electronics.

The present teachings encompass embodiments in other specific formswithout departing from the spirit or essential characteristics thereof.The foregoing embodiments are therefore to be considered in all respectsillustrative rather than limiting on the present teachings describedherein. Scope of the present invention is thus indicated by the appendedclaims rather than by the foregoing description, and all changes thatcome within the meaning and range of equivalency of the claims areintended to be embraced therein.

What is claimed is:
 1. A thin film transistor comprising: a thin film semiconductor; source and drain electrodes in contact with the thin film semiconductor; a dielectric layer adjacent the thin film semiconductor, wherein the dielectric layer comprises a thermally cured product of a blend composition comprising an inorganic oxide precursor sol and an organosilane crosslinker; and a gate electrode in contact with the dielectric layer; wherein the inorganic oxide precursor sol comprises one or more metal oxide precursors, and wherein the one or more metal oxide precursors are present in the blend composition at a molar concentration in excess of the organosilane crosslinker.
 2. The thin film transistor of claim 1, wherein the metal oxide precursors are selected from metal chlorides, metal alkoxides, and combinations thereof.
 3. The thin film transistor of claim 1, wherein the metal oxide precursors comprise a metal selected from Zr, Ti, Hf, and Ta.
 4. The thin film transistor of claim 1, wherein the inorganic oxide precursor sol comprises a solvent or solvent mixture comprising an alcohol.
 5. The thin film transistor of claim 1, wherein the inorganic oxide precursor sol comprises a hydrolyzing catalyst.
 6. The thin film transistor of claim 1, wherein the one or more metal oxide precursors are present in the blend composition at a molar concentration between about 2 times and about 10 times of the molar concentration of the organosilane crosslinker.
 7. The thin film transistor of claim 1, wherein the organosilane crosslinker is functionalized with hydrolysable groups.
 8. The thin film transistor of claim 1, wherein the organosilane crosslinker has the formula: (X)_(3-m)(Y)_(m)Si—Z—Si(Y)_(m)(X)_(3-m), wherein: m, at each occurrence, independently is selected from 0, 1, and 2; X, at each occurrence, independently is selected from a halide, an amino group, an alkoxy group, and a carboxylate group; Y, at each occurrence, independently is selected from H, an alkyl group, and a haloalkyl group; and Z is a divalent organic group comprising 1 to 20 carbon atoms.
 9. The thin film transistor of claim 1, wherein the thermally cured product is obtained at a curing temperature of less than about 300° C.
 10. The thin film transistor of claim 1, wherein the thin film semiconductor comprises an organic semiconducting molecule or polymer.
 11. The thin film transistor of claim 1, wherein the thin film semiconductor comprises an inorganic oxide semiconductor.
 12. The thin film transistor of claim 1, wherein the gate electrode is deposited on a flexible substrate.
 13. The thin film transistor of claim 1, wherein the dielectric layer has a film thickness between about 10 nm and about 50 nm, and exhibits a capacitance per unit area of at least about 90 nF cm⁻².
 14. The thin film transistor of claim 1, wherein the dielectric layer has a dielectric constant of about 4.0 or higher.
 15. The thin film transistor of claim 1, wherein the thin film transistor exhibits a mobility of at least about 0.1 cm²/Vs and a current on/off ratio of at least about 10³ at an operating voltage of about ±10 V or lower.
 16. The thin film transistor of claim 1, wherein the organosilane crosslinker is a bis(silyl)alkane comprising 4 to 10 carbon atoms.
 17. The thin film transistor of claim 16, wherein each silyl group of the bis(silyl)alkane is functionalized with at least one alkoxy group.
 18. A thin film transistor comprising: a thin film semiconductor; source and drain electrodes in contact with the thin film semiconductor; a dielectric layer adjacent the thin film semiconductor, wherein the dielectric layer comprises a thermally cured product of a blend composition comprising a zirconium oxide precursor sol and a bis(silyl)alkane; and a gate electrode in contact with the dielectric layer; wherein the zirconium oxide precursor sol comprises one or more zirconium oxide precursors selected from the group consisting of ZrCl₄, ZrOCl₂, and Zr(OR)₄, wherein each R independently is a C₁₋₆ alkyl group, and wherein the one or more zirconium oxide precursors are present in the blend composition at a molar concentration in excess of the bis(silyl)alkane.
 19. The thin film transistor of claim 18, wherein the one or more zirconium oxide precursors are present in the blend composition at a molar concentration between about 2 times and about 10 times of the molar concentration of the bis(silyl)alkane.
 20. The thin film transistor of claim 18, wherein each silyl group of the bis(silyl)alkane is functionalized with at least one alkoxy group. 